• Haemoglobin is a large globular protein composed of four polypeptide chains, each chain containing an iron group
• Each haemoglobin molecule can pick up four oxygen molecules
• This is a reversible reaction and forms oxyhaemoglobin:

4O₂ + Hb (Haemoglobin)  → HbO₈ (Oxyhaemoglobin)

• The binding of the first oxygen molecule alters the formation of the molecules and so the other three oxygen molecules are able to bind much faster than the first
• The reverse of this process happens when oxygen dissociates at the tissues and the last oxygen molecule is the hardest to remove

Carbon dioxide

• Waste carbon dioxide diffuses from tissues and into the blood following aerobic respiration
• There are three main ways in which carbon dioxide is transported around the body
• A very small percentage of carbon dioxide (~ 10 %) dissolves in blood plasma, forming H₂CO₃
• A much larger percentage (~ 70 %) of carbon dioxide dissolves in the cytoplasm of red blood cells
• Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water
  • Without carbonic anhydrase this reaction proceeds very slowly. The plasma contains very little carbon anhydrase hence H₂CO₃ forms much more slowly in plasma than in the cytoplasm of red blood cells
• Carbonic acid dissociates readily into H⁺ and HCO^3- ions :

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃^– + H⁺

• The increase in H⁺ concentration results in a decrease in blood pH, which alters the structure of haemoglobin, encouraging the dissociation of oxyhaemoglobin to release oxygen
  • This is beneficial – when levels of carbon dioxide are higher, rates of aerobic respiration are greater and therefore the need for oxygen is higher
• Hydrogen ions (protons) can combine with haemoglobin, forming haemoglobinic acid
• Carbon dioxide can also bind to amino acids and therefore haemoglobin, forming carbaminohaemoglobin – this accounts for ~ 20 % of carbon dioxide transport in the blood